This invention relates to an object detecting system of the reflecting type which utilizes ultrasonic waves, and more particularly to an object detecting system adapted to detect objects relatively near the system as well as those relatively far therefrom.
In motor vehicles, for example, there is known an object detecting system as a system for detecting obstacles locating out of a visual field of the driver and for guiding him. The object detecting system of this kind generally utilizes ultrasonic waves. More specifically, in this kind system ultrasonic waves of the given strength are emitted from a ultrasonic transmitter in the predetermined direction, and a ultrasonic receiver is set in the same direction. The presence of reflected waves, i.e., the presence of obstacles is detected based on strength and timing of the signal which is generated in the ultrasonic receiver.
Heretofore, when detecting objects at a distance ranging from several cm to several m by utilizing ultrasonic waves, there have been encountered problems conflicting with each other in the near and far ranges. More specifically, ultrasonic waves are attenuated so much in a space, i.e., in the atmosphere into which they are emitted, and an attenuation amount of ultrasonic waves is largely changed depending on the distance. Therefore, when the energy of ultrasonic waves emitted from the ultrasonic transmitter is set high for permitting detection of objects in the far range, direct waves, secondary reflected waves, etc. are also detected when detecting objects in the near range, thus resulting in a fear that there may occur an error in detection of objects. On the other hand, when the energy of the emitted ultrasonic waves is set low for avoiding influences due to direct waves, secondary reflected waves, etc. in the near range detection, energy of the reflected waves becomes too small to detect objects in the far range. In particular, when direct waves are detected in the near range measurement, a time lag between the reflected waves from objects and the direct waves is very small, so that it is extremely difficult to discriminate one from another. In the past, therefore, such object detecting system was designed to have a property adapted for the near range detection or that adapted for the far range detection, and the system having either one property or the other was selected in accordance with the use.
But when the system is employed in motor vehicles for detecting obstacles, for example, it is required to detect the presence of objects in the near range with the vehicle running at a relatively low speed in such a case where the vehicle is driven to enter into a garage, and it is also required to detect the presence of objects located fairly far away with the vehicle running at a relatively high speed. The use of plural detecting systems to meet both such requirements will result in increased cost and will need a large space for mounting the systems.
Meanwhile, if there occurs an anomaly on the transmitter side and ultarsonic waves are not generated from the transmitter, no ultrasonic wave reach the receiver even with obstacles being in existence actually. In this case, therefore, the system makes a judgment that there exists no obstacle and provides the driver with erroneous information, thus causing the possibility of danger. For example, when the motor vehicle is driven backward while checking the presence or absence of obstacles by the use of detectors provided on the rear portion of the vehicle, the driver believes that no obstacle exists there, if any failure has been occurred in the system. Thus, in the event there actually exists some obstacle, the vehicle may collide against the obstacle. In the prior system, for this reason, it is necessary for the driver to place any optional object in front of the object detector, thereby to check whether the system operates normally or not. However, to make such a check is very troublesome.